1. Field of the Invention
This invention relates to variable magnification optical systems having a function of stabilizing a photographic image that has moved by the vibration of the camera and, more particularly, to variable magnification optical systems having the image stabilizing function which has achieved minimization of the size and weight of the compensating lens unit for image stabilization and to prevent a lowering of optical performance when the compensating lens unit is moved, for example, in a direction perpendicular to the optical axis to play role of the image stabilizing effect and also has achieved improvement of the control performance of the actuator.
2. Description of the Related Art
When shooting from a moving car, flying aircraft, or other moving body, vibrations are propagated to the photographic system and deviation of the camera's image from the line of sight takes place.
Vibration-proof optical systems having the function of preventing deviation of the camera's image have been proposed in, for example, Japanese Laid-Open Patent Application No. Sho 50-80147, Japanese Patent Publication No. Sho 56-21133 and Japanese Laid-Open Patent application No. Sho 61-223819.
In the document of Japanese Laid-Open Patent Application No. Sho 50-80147, the zoom lens having two afocal magnification varying units performs variation of the image magnification in such a way that the angular magnification M.sub.1 of the first magnification varying unit and the angular magnification M.sub.2 of the second magnification varying unit have the relation of M.sub.1 =1-(1/M.sub.2) and the second magnification varying unit is operatively connected to the gimbal mechanism to compensate for deviation of the image, thus achieving stabilization of the image.
In Japanese Patent Publication No. Sho 56-21133, stabilization of the image is achieved by moving a different optical member from the zoom units to such a direction as to cancel the vibratory displacement of the image due to the vibration depending on the output signal of detecting means for detecting the vibrated state of the optical instrument.
Japanese Laid-Open Patent Application No. Sho 61-223819 discloses that in the photographic system having a refraction type variable vertical angle prism arranged at the frontmost position, the vertical angle of that refraction type variable vertical angle prism is made to vary in correspondence to the vibration of the photographic system, causing the image to deflect. Thus, the stabilization of the image is achieved.
Besides these, Japanese Patent Publications Nos. Sho 56-34847 and 57-7414 disclose that an optical member constituting part of the photographic system is spatially fixed by using a gyro or like mechanism to produce a prism effect as the instrument vibrates. By utilizing this effect, the instrument's image is deflected to obtain a stabilized image on the focal plane.
Also, another method of obtaining the stabilized image is even adopted wherein by utilizing the acceleration sensor, the vibration of a photographic system which is constructed from the single focal length is detected. In response to the thus-obtained signal, part of the photographic system, or a lens unit, is made to vibrate in a direction perpendicular to the optical axis. As its examples there are U.S. patent application Ser. No. 116,541 filed Nov. 4 1987, now U.S. Pat. No. 4,844,602 Ser. No. 116,668 filed Nov. 4, 1987, now abandoned Ser. No. 156,930 filed Feb. 17, 1988, now U.S. Pat. No. 4,907,868 and Ser. No. 896,639 filed Aug. 15, 1986 now abandoned.
In general, the mechanism for obtaining the stabilized image by vibrating the lens unit of part of the photographic system to remove the deviation of the photographic image is required to have a good responsiveness.
For this reason, the size and weight of the compensating lens unit must be as far minimized as possible and the inertial mass must be reduced. Further, the relation between the amount of compensation for the deviation of the image and the amount of movement of the compensating lens unit must be so much simplified as to shorten the computation time for their conversion. When these requirements are fulfilled, realization of the desired photographic system is achieved.
Also, when the compensating lens unit is decentered, decentering coma, decentering astigmatism and decentering curvature of field are produced to large degrees. Hence the compensated image to the deviation has its sharpness decreased by the decentering aberrations. For example, when large decentering distortion is produced, the amount of movement of the image on the optical axis and the amount of movement of the image in the marginal zone become different from each other. From this reason, when the compensating lens unit is decentered to compensate for the deviation of the image in respect to the paraxial region thereof, it, is in the marginal zone that a similar phenomenon to the defocusing of the image comes to appear, becoming a cause of extremely lowering the optical performance.
Thus, in the photographic system for image stabilization, particularly the variable magnification optical system, it is required that when the compensating lens unit is moved in a direction perpendicular to the optical axis to a decentered state, the amounts of decentering aberrations produced are little and the lowering of the optical performance is little.
However, it is generally very difficult to obtain a variable magnification optical system which has satisfied all the above-described various conditions. Particularly when the lens unit having a refractive power which constitutes part of the photographic system is decentered, the optical performance lowers largely, thus giving rise to a drawback that a good image cannot be obtained.